Research Study Finds Fossil Sex Differences More Nuanced Than Thought|Mirage News

LAWRENCE– For years, paleontologists have actually held up uintatheres as a prime illustration of sexual dimorphism, or the propensity for males and women of a types to be physically various.

Noteworthy for their plus size, popular horns and canine teeth, uintatheres emerged about 58 million years earlier, less than 10 million years after an area rock eliminated the dinosaurs, and appear in the fossil record for the next 20 million years approximately.

a research study from the University of Kansas released today in the journal PLOS One reveals the distinctions in between male and female uintatheres may not be as large as paleontologists have actually long thought.

“Males have been interpreted as being much larger and possessing far more dramatic cranial features than females,” stated research study author Kevin Mulcahy, doctoral trainee at the KU Biodiversity Institute & & Natural History Museum.

Mulcahy and his doctoral consultant, K. Christopher Beard, senior manager at the institute, were attempting to recognize a single damaged upper molar from fieldwork in Montana that the scientists thought might have come from the uintathere group.

“We had a research visit at the American Museum of Natural History in New York, where our main goal was just to compare that tooth to teeth from other uintatheres and figure out what type of animal we had,” Mulcahy stated. “They have this big collection — entire shelves occupied with these skulls, and each one of them is almost a meter long. These are famous specimens that are in the literature, and I was originally just looking at them to compare their teeth, and I noticed a ton of variation in a way that didn’t appear binary.”

The KU scholar started tape-recording measurements of the skulls at the AMNH.

“I just, really hastily, with a ruler, gathered a bunch of measurements of the skulls while we were there,” Mulcahy stated. “I started playing around with statistics, and there are ways you can look at different measurements to see if they’re best fit by a classic unimodal distribution, or if you were to think you’re dealing with something that exhibits strong sexual dimorphism, you’d see something with a bimodal distribution — that there are two means, one for males and one for females.”

After analysis, the KU scientist wasn’t seeing that circulation in the qualities he ‘d determined in New York.

“It wasn’t like there’s one group of specimens that look bigger than another, and then there’s another group that are smaller with smaller horns and canines,” he stated. “So I was kind of interested — are these really good examples of sexual dimorphism after all? Is there actual evidence in the morphology of these skulls, in the distribution in particular of the ranges of morphology we’re seeing, that that’s what’s going on?”

Mulcahy branched off, collected more measurements from specimens at other organizations, and utilized brand-new analytical treatments. In the end, his dataset consisted of about 30 skulls of one extinct types, Uintatherium anceps.

“Maybe there were some little signals of dimorphism here and there, but nothing really concrete,” he stated. “I had an idea: Maybe it’s just that the techniques I’m using aren’t robust enough to actually measure sexual dimorphism. With fossil animals, you don’t actually know which specimens are males and which are females. You can guess and then test whether or not the ones you guess are males differ from the ones you guess are females, but that’s super circular, because you could just be picking the most extreme ones to be one sex and the least extreme to be the other. Of course they’re going to differ.”

The KU scientist chose to utilize the very same analytical methods he was utilizing on uintatheres on modern-day bison, as he might be sure of the sex of each specimen.

“When you’re dealing with living animals, you can just take the males, take the females and compare them,” he stated. “Bison are another really big herbivore, with a similar sort of ecology that’s been proposed for uintatheres, and they also have big horns. What I noticed is that the signals of dimorphism I was picking up in bison were a lot clearer. When I was looking at the bison, it was almost perfect that all of the females were most similar to the females, and all the males were most similar to the males.”

Mulcahy stated incorrect presumptions about sexual dimorphism might have roots in the cultural predispositions of the late 19^th and early 20^th century, when many fossil types were very first explained.

“Back then, this is right when evolutionary theory, sexual selection, natural selection, all of that is kind of coming into play,” he stated. “I think it’s really important to remember that these things were first discovered in the late 1800s by a bunch of male paleontologists working in a very Victorian-era mindset. The idea is basically assumed at that point that males are going to be bigger and scarier than females. It’s really interesting: When you step back and actually test that hypothesis, it doesn’t really hold up.”

The KU scientist stated a weak signal for dimorphism in uintatheres, long a flagship for dimorphism, recommends other types in the fossil record may likewise be less dimorphic than paleontologists have actually held.

“Strong, big male-style sexual dimorphism was probably not ancestral at the root of the mammalian tree, and among the mammals where we do see this pattern today, it’s almost certainly shown up independently several times,” Mulcahy stated. “It had long been assumed that sexual dimorphism is the norm, that most species show that males are bigger than females. But just a couple years ago, a paper came out showing that among living mammals, most species either don’t show any dimorphism or actually have larger females. The idea that males are always bigger than females is really just a choice of what animals were people looking at, what are the things that biologists in the past were interested in.”

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